1. Field of the Invention
The present invention relates to a creep control device for a vehicle for controlling the degree of engagement of a lock-up clutch provided in a torque converter for an automatic transmission when the throttle angle is nearly zero, the vehicle is in a substantially stopped condition, and the shift position is in a running range, thereby controlling the creep torque to be transmitted to wheels.
2. Description of the Related Art
In an automatic transmission having a torque converter, the torque from the torque converter driven by engine idling is transmitted to wheels even when the throttle is fully closed in a running range, thus allowing creep running. In the condition where the vehicle is kept stopped by depressing a brake pedal, the creep torque is unnecessary. Accordingly, in this case, the engine consumes a redundant amount of fuel corresponding to the amount for producing the creep torque, causing a reduction in fuel economy. Further, there arises another problem such that unpleasant engine vibrations are produced in association with an increase in engine load.
Various creep control devices for a vehicle having an automatic transmission have conventionally been proposed to reduce the fuel consumption by partially engaging a lock-up clutch provided in a torque converter in the condition that the shift position is in a running range, that the throttle is fully closed, and that the vehicle is kept stopped or substantially stopped, thereby reducing the load on the engine during idling. For example, Japanese Patent Laid-open No. 2001-116067 discloses a creep control device for a clutch in a vehicle having an automatic transmission capable of performing creep running, wherein two strong and weak creep forces are set and the degree of engagement of the clutch provided in a power transmitting system for transmitting engine power to wheels is controlled to thereby switch between these strong and weak creep forces.
In this creep control device, the clutch is controlled to produce the weak creep torque in the condition where the vehicle is kept stopped by depressing a brake pedal (which control will be hereinafter referred to as weak creep control), thereby allowing a reduction in fuel consumption corresponding to the redundant creep torque produced in the case where no creep torque is required. Further, in the case that the slope of a road surface on which the vehicle is running is greater than or equal to a predetermined value, the weak creep control is not performed to thereby prevent minute backward movement of the vehicle at starting on the upward sloping road surface.
However, in the creep control device disclosed in Japanese Patent Laid-open No. 2001-116067, a redundant creep force is produced on a road surface having a slope range between a lower slope where the weak creep control is inhibited and a higher slope where the strong creep torque becomes a necessary and sufficient torque for prevention of the minute backward movement of the vehicle at starting. Accordingly, an unnecessary amount of fuel corresponding to this redundant creep force is consumed.